Typically, polymeric materials and organic materials used for forming plastics, rubbers, paint coatings, etc. are characterized by including macromolecules as a main component to which organic elements such as carbon, hydrogen, nitrogen, oxygen, etc. are covalently bonded, and have properties of relatively weak degradation by the degradation factor from light, heat, etc. compared to metal materials and ceramic materials. Physical degradation phenomenon such as creep, fatigue and stress relaxation is mostly based on destruction of bond between molecules or movement between molecules. On the contrary, chemical degradation by light such as UV is mostly companied with destruction of bond within molecules.
The degradation of the polymer by UV light is generated when the polymer absorbs the light energy corresponding to the energy destructing the main chain of the polymer. Here, the energy of destruction is called the bond dissociation energy.
The bond dissociation energy depends on a variety of molecule bond states from the weak bond energy such as a bond (O—O) of peroxide structure to the strong bond energy such as a bond (O—H) of hydroxyl group. In case of carbon-carbon bond or carbon-hydrogen bond constituting a main chain of molecule, the bond dissociation energy has a relatively high energy more than 83 kcal/mol.
Thus, a polymeric material consisting of only carbon-carbon bond or carbon-hydrogen bond such as polyethylene and polypropylene is expected to provide a relatively better light resistance. In fact, however, the light resistance may not be reached to the expectation. This result is estimated to be caused by a number of reasons. However, a problem of molecular structure due to carbon-carbon bond in the position of allyl located at an irregular defect structure or carbon-hydrogen bond of the tertiary carbon located at a chain structure such as vinyl and vinylidene structures formed by repeating irregular reaction or side reaction of monomer of polymerization may be issued to be a reason for a relatively lower dissociation energy. Accordingly, even in the polymer materials represented by the same molecular structure, light resistance may differ depending on kinds or amounts of such internal defects.
The difference of photo-degradation depending on the difference of the aforementioned molecular structure may occur even when molecular structures of a polymeric material or an organic material are different against the same light, but also is to be meant to occur even when conversely, light characteristics being applied to a polymeric material or an organic material having the same molecular structure are different. Thus, in the accelerated photo-degradation test or a weathering test, depending on how much the light characteristic used for the test apparatus is consistent with the actual solar light characteristic, it becomes the most important technical characteristics how much the photo-degradation phenomenon by the solar light outdoor can be reproduced or accurately predicted.
Radiant energy sending out from the sun to the surface of the earth contains a variety of light of wavelengths ranging from UV light, visible light and to infrared light. Such a division of the component of the solar light is merely separated to UV light, visible light and infrared light based on a specific wavelength band, and does not mean that such three types of different lights reach to the surface of the earth respectively. In fact, the solar light has a continuous spectrum over a wide wavelength band, and does not have any criteria to separate the solar light into such three kinds of lights.
The surface temperature of the sun is typically known to be 5,780 K, and light power spectrum of the solar radiation has a similar distribution of wavelength with a continuous spectrum of the blackbody also having a temperature of 5,780 K. However, the solar radiation is subject to reflection, scattering, absorption, etc. during passing through the earth's atmosphere. Therefore, the solar radiation energy passing through the earth's atmosphere is not consistent with the solar radiation energy reaching to the stratosphere outside the earth's atmosphere.
A wavelength band showing the most significant change is a short-wavelength UV light range of which most are filtered in the ozone layer in the earth. Therefore, UV light with a short-wavelength range of about 200-295 nm existing outside of the atmosphere is mostly removed, and the solar radiation energy reaching to the earth's surface contains light with a wavelength more than about 295 nm only.
Because of such characteristics, the amount of the short-wavelength UV light contained in the solar radiation energy is affected even by a way passing through the atmosphere, and also depending on an altitude of the sun from sea level and an altitude of the sun to the earth' surface, the effect on passing through the atmosphere appears differently.
When the solar radiation energy reaches to the earth's surface, as the light power spectrum of the solar light is affected by area and climate, the amount of UV light contained in the solar radiation and light power characteristics vary depending on area, season, weather and a time zone. For convenience sake, however, a certain reference is standardized for using. According to the Publication CIE 85-1989 which was established by International Commission on Illumination (CIE) as a reference for the standard light source for simulating the solar light for the test purpose, content ratio of UV light, visible light and infrared light in the solar radiation is typically 6.8%, 55.4% and 37.8%, respectively.
In the UV light contained in the solar radiation reaching to the earth' surface, content of the UV light with a short-wavelength of UVB region (about 280-315 nm) corresponding to the shortest wavelength is merely about 3%. However, as the light included in the UVB region has relatively strong light photon energy, even if the amount of the light in the UVB region is small, it can exert a strong effect on the photo-degradation of polymer and organic materials, thereby considering as an important artificial light source for the solar light simulation.
In order for the UV light exposed into an organic material such as a polymeric material to exert an effect on the chemical degradation, the energy of UV light exposed into the material must take place to be absorbed, which essentially depends on a unique UV light absorbing characteristic of the organic material. Such a characteristic is basically dependent on chemical molecular structure, and different absorbing characteristics are provided for each polymeric material. Typically, aspects of the UV absorption spectrum of many polymeric materials show trends that absorption rate at a short-wavelength region near 300 nm is increased and absorption rate at a long-wavelength region near 400 nm is decreased. As aspects are different for each polymeric material, however, it is difficult to generalize.
When a light source having a specific light power spectrum such as the solar light is irradiated to the polymeric material having different UV light absorption spectrum as above mentioned, the coupler of the polymeric material absorbs the light energy of the specific wavelength to make a chemical degradation to be progressed resulting in decomposition and oxidation reactions. Accordingly, an activated spectrum for the specific light source such as the solar light becomes to exist.
A variety of the activated spectrum caused by differentiated UV light absorption spectrum for each polymeric material consequentially becomes to a main reason causing an inconsistency between tests of photo-degradation by other light source with different light power spectrum in the UV light or the visible light region. This is the most fundamental reason causing an inconsistency between the test result with an artificial light source inconsistent with the light power spectrum of the solar light and the test result with the actual solar light. In particular, it is important whether in the UV light region exerting a direct effect on the photo-degradation, the light power spectrum between the solar light and the artificial light is inconsistent. Accordingly, in the photo-degradation test for the polymeric material and the organic material, it becomes the significance technically to select an artificial light source and also to use a filter with a function of removing UV light in a specific wavelength region.
However, there is no light source available having a light power spectrum fully consistent with the solar light in the developed artificial light sources so far. Even a xenon-arc lamp or a metal-halide lamp commonly employed for simulation for the solar light is not possible to provide a consistency with the solar light without using a filter, and even in case of using an appropriate filter, it is not possible to perfectly consist with the solar light power spectrum in the entire region as well as in the UV light region. Therefore, a realistic choice is taking a method to use a UV light filter capable of removing a short-wavelength UV light not contained in the solar light so as to consist with the solar light, even though the short-wavelength UV light severely is affected on the photo-degradation.